Under cover of darkness, thieves dove into the inky waters of Tennessee’s river sanctuaries and scooped up endangered washboard mussels by the thousands. The thick shells of these animals—some as big as Frisbees—

were destined to be cut into cubes, polished round, and implanted in saltwater oysters to grow cultured pearls. “Every one of them was almost a twenty-dollar bill,” says David Sims, then a game warden with the Tennessee Wildlife Resources Agency who was charged with surveying these bivalves. Poachers could wipe out an entire bed of mussels in a single night.

Today, 25 years later, Sims can rattle off the common names of freshwater mussel species like a lineup of Seussian characters: Appalachian monkeyfaces, white wartybacks, Tennessee heelsplitters, fat pocketbooks, shiny pigtoes, pistolgrips, and spectaclecases. The orange-footed pimpleback conceals in its bumpy case a fleshy appendage the color of Orange Crush soda. “They’re hard to find,” Sims says, lifting a glistening mussel from a gurgli

ng tank. “We hold on to them, once we’ve got them.” He cradles the creature like a rare jewel as the mussel pulls the two halves of its shell closed and an elegant arc of water shoots out.

North America hosts the richest variety of freshwater mussels in the world, and the epicenter for this biodiversity is in the southeastern United States. In surveys, biologists have found more species in a few square meters of the Tennessee River than are found in all of Europe. The smallest mussel species can fit on a pinky fingernail when fully grown; the largest have the diameter of a dinner plate. Their shells grow rings over time, like trees, and some can live to be more than a hundred years old.

Mussels are critical, if unsung, players in river systems. They spend their lives half-buried in sediment, stabilizing riverbeds and preventing erosion. There they serve as food for birds, muskrats, and otters; and when they die, their shells are adopted as secondhand homes by crayfish and aquatic insects. Most importantly, mussels filter river water—up to 20 gallons per mussel per day by some estimates. They live off the algae and plankton gleaned from this filter feeding, and in the process remove silt and toxic substances from the water, including some heavy metals, harmful bacteria, pharmaceuticals, and endocrine-disrupting chemicals such as PCBs. “They’re like little miniature treatment plants,” says stream ecologist Jesse DiMartini, of the DuPage County Forest Preserve District in Illinois.